i’m falling in love with my bestfriend’s brother

I need everyone’s advice right now because i am just completely drawing a blank and i have no idea what to do. so basically i have the BIGGEST crush ever on my best friend’s brother. My best friend is so sweet but she sometimes gets weird about attention and how it always need to be on her so if i ever told her about this she would flip her shit on me. Also i forgot to mention that my friend’s brother likes me back!! I really want to hookup with him and have a sort of secret fling but i’m just not sure because i don’t want to ruin my friendship with my best friend. I would totally forget about my friend’s brother tho if I didn’t have actual feelings for him. Whenever I see him he stares right into my eyes and gives my the cutest little smile!! I just can’t he’s the most adorable thing I have ever seen and I just don’t think I can keep my feelings contained. he’s so sweet to me and always tries to find little ways to touch and talk to me. I snapchat him all the time and we talk now more than i talk to my friend!! Ok as u can see im absolutely infatuated with this guy but also stuck in a huge dilemma so pls help a girl out and comment below!!!